Dizygotic mosaic animals (chimeras, allophenics) are experimentally produced by amalgamating preimplantation embyronic tissues. The combined embryos are removed to the reproduction tract of pseudopregnant surrogate mothers. Adult tissues or the offspring are a mosaic of the progeny of cells derived from one of the other of the genotypes originally mixed. If the cell types are distinguishable in some way, then the mosaicism can be demonstrated. Most of the developmental studies utilizing chimeric animals are characterized by biochemical markers for which tissues are homogenized and then analyzed by electrophoresis. McLaren has stated that the ideal marker of chimerism would be cell autonomous, ubiquitously expressed and useful in histologic section. Iannaccone and associates have recently developed such a system for the rat. Embryos of congenic strains of rat varying in expression of class I major histocompatibility antigens are combined by aggregation. The resulting chimeric tissues are distinguishable in histological section with iodinated monoclonal antibodies directed to the variant major histocompatibility antigens. The widespread usefulness of such a system in developmental studies appears certain. With the assistance of Dr. Iannaccone, BIOQUAL proposes to makes this innovative research tool, namely "custom developed" standardized chimeric rats, available to the scientific community.